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Subject: Re: [PATCH] mm/filemap: add static for function __add_to_page_cache_locked
From:   Greg Thelen <gthelen@google.com>
To:     Alex Shi <alex.shi@linux.alibaba.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Souptick Joarder <jrdr.linux@gmail.com>
Cc:     Linux-MM <linux-mm@kvack.org>, linux-kernel@vger.kernel.org,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Josef Bacik <josef@toxicpanda.com>
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Alex Shi <alex.shi@linux.alibaba.com> wrote:

> =E5=9C=A8 2020/11/11 =E4=B8=8A=E5=8D=883:50, Andrew Morton =E5=86=99=E9=
=81=93:
>> On Tue, 10 Nov 2020 08:39:24 +0530 Souptick Joarder <jrdr.linux@gmail.co=
m> wrote:
>>=20
>>> On Fri, Nov 6, 2020 at 4:55 PM Alex Shi <alex.shi@linux.alibaba.com> wr=
ote:
>>>>
>>>> Otherwise it cause gcc warning:
>>>>           ^~~~~~~~~~~~~~~
>>>> ../mm/filemap.c:830:14: warning: no previous prototype for
>>>> =E2=80=98__add_to_page_cache_locked=E2=80=99 [-Wmissing-prototypes]
>>>>  noinline int __add_to_page_cache_locked(struct page *page,
>>>>               ^~~~~~~~~~~~~~~~~~~~~~~~~~
>>>
>>> Is CONFIG_DEBUG_INFO_BTF enabled in your .config ?
>>=20
>> hm, yes.
>
> When the config enabled, compiling looks good untill pahole tool
> used to get BTF info, but I still failed on a right version pahole
>> 1.16. Sorry.

I'm seeing an issue with this patch.  My build system has pahole v1.17,
but I don't think the pahole version is key.

$ git checkout 3351b16af494  # recently added to linus/master
$ make defconfig
$ make menuconfig  # set CONFIG_BPF_SYSCALL and CONFIG_DEBUG_INFO_BTF
$ make V=3D1
+ ./tools/bpf/resolve_btfids/resolve_btfids vmlinux
FAILED unresolved symbol __add_to_page_cache_locked

Reverting 3351b16af494 ("mm/filemap: add static for function
__add_to_page_cache_locked") fixes the issue.

I don't see the warning which motivated this patch, but maybe it
requires particular a .config or gcc version.  Perhaps adding a
__add_to_page_cache_locked() prototype would meet avoid it.  But I
haven't studied enough on BTF to know if there's a better answer.

>>>> Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
>>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>>> Cc: linux-mm@kvack.org
>>>> Cc: linux-kernel@vger.kernel.org
>>>> ---
>>>>  mm/filemap.c | 2 +-
>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/mm/filemap.c b/mm/filemap.c
>>>> index d90614f501da..249cf489f5df 100644
>>>> --- a/mm/filemap.c
>>>> +++ b/mm/filemap.c
>>>> @@ -827,7 +827,7 @@ int replace_page_cache_page(struct page *old, stru=
ct page *new, gfp_t gfp_mask)
>>>>  }
>>>>  EXPORT_SYMBOL_GPL(replace_page_cache_page);
>>>>
>>>> -noinline int __add_to_page_cache_locked(struct page *page,
>>>> +static noinline int __add_to_page_cache_locked(struct page *page,
>>>>                                         struct address_space *mapping,
>>>>                                         pgoff_t offset, gfp_t gfp,
>>>>                                         void **shadowp)
>>=20
>> It's unclear to me whether BTF_ID() requires that the target symbol be
>> non-static.  It doesn't actually reference the symbol:
>>=20
>> #define BTF_ID(prefix, name) \
>>         __BTF_ID(__ID(__BTF_ID__##prefix##__##name##__))
>>=20
>
> The above usage make me thought BTF don't require the symbol, though
> the symbol still exist in vmlinux with 'static'.
>
> So any comments of this, Alexei?=20
>
>> Alexei, can you please comment?
>>=20